Wash the spinach well and chop roughly.
Without adding any water, place the spinach in a pot and cook until wilted. Drain and set aside.
In a large skillet, heat the olive oil over a medium flame.
When hot, add the onions and cook until they are clear.
Add the cooked spinach, dill, parsley and salt, and cook uncovered for about 10 minutes or until most of the liquid in the skillet has evaporated. Stir occasionally.
Transfer the spinach to a bowl.
When cool, mix in the milk, feta cheese and eggs. Set aside.
Preheat the oven to 375°F. With a pastry brush, coat the bottom and sides of a 10 x 14" inch baking dish with melted butter. Line the dish with a sheet of filo and then brush the entire surface with melted butter.
While working with the filo dough, cover the unused sheets with a slightly damp towel to prevent them from drying out.
Repeat this process until you have used 8 sheets of filo in all. Spread the spinach mixture into the dish.
Cover with the remaining 8 layers of filo following the same procedure as above.
Brush the top layer with melted butter and bake in the center of the oven for 30 minutes, or until the crust is golden brown.
Remove from the oven and let cool 15 minutes before cutting.